As carbon dioxide levels in the atmosphere continue to climb, most climate models project that the world’s oceans and trees will keep soaking up more than half of the extra CO2. But researchers report this week that the capacity for land plants to absorb more CO2 will be much lower than previously thought, owing to limitations in soil nutrients1.
Because plants take up CO2 during photosynthesis, it has long been assumed that they will provide a large carbon ‘sink’ to help offset increases in atmospheric CO2 caused by the burning of fossil fuels. Some scientists have argued that the increase might even be good for plants, which would presumably grow faster and mop up even more CO2. Climate models estimate that the world’s oceans have absorbed about 30% of the CO2 that humans have released in the past 150 years and that land plants have gulped another 30%.
But the latest study, by ecologists Peter Reich and Sarah Hobbie at the University of Minnesota in St Paul, suggests that estimates of how much CO2 land plants can use are far too optimistic. Plants also need soil nutrients, such as nitrogen and phosphorus, to grow. But few studies have tested whether soils contain enough of these nutrients to fuel growth in proportion to rising CO2.
“This work addresses a question that’s been out there for decades,” says Bruce Hungate, an ecosystem scientist at Northern Arizona University in Flagstaff. "It's a hard question to answer, because it takes a long time to see how ecosystem carbon and nitrogen cycles change."
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